Method of and apparatus for monitoring a ball forming process

ABSTRACT

A method of monitoring a ball forming process in a wire bonder. A glow discharge parameter between an electrode on the wire bonder and an end of a wire mounted on the wire bonder on which the ball is being formed is monitored. The monitored parameter is compared with a reference value to determine whether the ball formed is satisfactory.

The invention relates to a method of and apparatus for monitoring a ballforming process and in particular, a ball forming process where a ballof metal is formed at the end of a wire in a wire bonder.

During assembly of semiconductor packages, a thin metal wire isultrasonically welded to a contact pad on a semiconductor die and to acorresponding contact on a leadframe on which the die is mounted toestablish an electrical connection between the pad and the correspondingcontact on the leadframe. This is commonly known as the wire bondingprocess and is carried out using a wire bonder. The wire is held in thewire bonder in a work piece known as a capillary and the wire extendsthrough a central bore of the capillary. The capillary holds the wireduring bonding and couples the ultrasonic energy to the wire toultrasonically weld the wire to the contact on the die (commonly knownas the die pad) or the contact on the leadframe.

Normally, the end of the wire is bonded to the die pad before the wireis bonded to the contact on the leadframe. Prior to bonding the wire tothe die pad, a ball of metal is formed at the end of the wire by a glowdischarge between the end of the wire and an electrode on the wirebonder. The glow discharge causes the tip of the wire to melt andsurface tension effects cause the molten metal to form a ball on the endof the wire. It is important that the ball that is formed is of thecorrect size to ensure a satisfactory bond to the die pad. After theball is formed, the wire is pulled back up into the capillary so thatthe ball is at the capillary tip. This is to ensure rigidity between theball and the capillary tip and to enable efficient coupling ofultrasonic power from the capillary to the ball during the ultrasonicwelding process. Under normal conditions, a proper ball will be formedto a size determined by the current during the glow discharge and theduration of the glow discharge.

However, it is possible that either the wire or the electrode may becontaminated by substances that cause an irregular glow discharge. Forexample, the electrode may be contaminated by using it for a very longtime, such that substances are deposited at the end of the capillarytip, or by touching it with bare fingers or other objects. The wire maybe contaminated by a contaminated wire path, insufficiently purified airor by substances which are picked up from the substrate on which thesecond bond is made, especially when the substrate is of a non-metallicnature. Although the die pad is metallic, this may be contaminated bynon-metallic substances, such as PCB material. Such contamination mayresult in variations of the ball size. In addition, when the length ofwire protruding from the capillary tip is too short or too long, or whenthe wire end is bent at an irregular angle, the ball size may also beaffected.

When the ball is too small, the edge of the capillary will leave a markon the die when it touches down. This is normally referred to as a“capillary mark” or, in short, a “cap mark”. Devices with cap marksnormally have to be discarded, as the proper functioning of thatparticular die cannot be guaranteed. This causes loss of yield and may,especially for very large scale integration (VLSI) devices, result inappreciable financial loss.

In accordance with the present invention, a method of monitoring a ballforming process in a wire bonder comprises monitoring the glow dischargebetween an electrode on the wire bonder and an end of a wire mounted onthe wire bonder on which the ball is being formed, and comparing aparameter of the glow discharge with a reference value.

In accordance with a second aspect of the present invention, apparatusfor monitoring the formation of a ball on an end of a wire in a wirebonder comprises a detector to detect a parameter of a glow dischargebetween an electrode on the wire bonder and the end of a wire during theball formation process, and a processor coupled to the detector toreceive an output signal from the detector, the processor comparing theoutput signal with a reference value.

An advantage of the invention is that by monitoring a parameter of theglow discharge, it is possible to obtain an indication of whether theball formed is of the correct size.

Preferably, the parameter is the glow discharge voltage. However, it ispossible that the parameter could be the glow discharge current.

In one example of the invention, the ball formation process is dividedinto a number of time intervals and the glow discharge parameter duringeach time interval is integrated over the time interval, and for eachtime interval each integrated value is compared with a reference value.

Preferably, the processor may also determine the mean value of the glowdischarge parameter during the ball formation process and compare themean value with a reference value.

The method comprises the step to stop the automatic bonding process andprovide information to the operator about the nature of the fault if theprocessor detects that the glow discharge parameter is out of areference value. Alternatively, a dummy bond may be performed, in whichcase the automatic bonding process does not need to be interrupted.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of a method of and apparatus for monitoring formation of aball at the end of a wire in a wire bonder will now be described withreference to the accompanying drawings, in which:

FIG. 1 is a block diagram of apparatus for monitoring formation of aball;

FIG. 2 is a graph of voltage versus time for formation of an ideal ball;and

FIG. 3 is a graph of voltage versus time for formation of a non-perfectball.

FIG. 1 shows a wire 5 that is connected to an output from a voltagedetector (not shown). The wire 5 is coupled to a two-way switch 4 thatis controlled by a processor 3 via a control wire 2. The processor 3 mayswitch the switch 4 to either a contact 6, which is coupled to a firstintegrator 8, or to a contact 7, which is coupled to a second integrator9. Both the integrators 8, 9 have outputs, which are coupled to ananalog digital converter (ADC) 10 and the output from the ADC 10 iscoupled to the processor 3. The processor 3 can also reset theintegrators 8, 9 via reset lines 11, 12 respectively. The processor 3 isalso coupled to an electrically erasable programmable read only memory(EEPROM) 13 and the processor 3 includes a random access memory (RAM)14. An output 15 from the processor 3 is coupled to a communicationinterface (not shown) which permits the processor 3 to communicate witha main processor (not shown) for a wire bonder on which the apparatus 1is used to monitor the formation of a ball at the end of a wire in acapillary on the wire bonder.

A typical ideal voltage versus time waveform for a ball formationprocess is shown in FIG. 2. Initially, a negative voltage is appliedbetween a discharge electrode on the wire bonder and the end of the wirein the capillary in the wire bonder and the negative voltage isincreased until breakdown occurs. The typical breakdown voltage (BB) isnormally in the region of approximately −3 kV to −4 kV but can be ashigh as −5 kV. When breakdown occurs, the voltage immediately drops, asa glow discharge is setup between the end of the wire and the electrode.The typical glow discharge voltage (V_(G)) is normally approximately−400 V with a corresponding current of approximately −30 mA for normalgauge wire, which is typically 25 μm to 32 μm diameter. For heavy-dutywire, the current may be up to 200 mA for wire having a 75 μm diameter.The glow discharge voltage (V_(G)) is sustained for a given timeinterval (T_(O)) to form a ball at the end of the wire, the voltage isthen returned to zero to stop the glow discharge process.

As shown in FIG. 2, the glow discharge time interval (T_(O)) is splitinto a number of time intervals (T_(S)) 20. The processor 3 controls theswitch 4 so that the switch 4 is coupled to the contact 6 and thecontact 7 alternatively, for each of the time intervals (T_(S)).Therefore, for each time interval (T_(S)), the respective integrator 8,9 integrates the output voltage 5. At the end of the time interval, theprocessor 3 switches the switch 4 to the other contact 7 and theintegrator 8 ouputs the result of the integration to the ADC 10 forconversion to a digital signal which is then output to the processor 3.After the processor 3 receives the digitally converted output from theintegrator 8, the processor 3 resets the integrator 8 using the resetline 11. While this is happening, the integrator 9 has integrated thevoltage (V_(G)) over the next time interval 20 and at the end of thenext time interval 20, the processor 3 switches the switch 4 back to thecontact 6 and the integrator 9 outputs the result of the integration tothe ADC 10 for conversion to a digital signal which is then output tothe processor 3, and the processor 3 then resets the integrator 9. Thisprocess continues until the end of the glow discharge period.

When the glow discharge is finished, the processor 3 compares each ofthe integrated voltages for each time interval 20 with a reference valueand also calculates the mean value of the integrated voltages andcompares this with another reference value. If the actual mean value orany of the individual integrated values are out with a predetermineddeviation from the reference values, the processor 3 outputs an errormessage on the serial link 15 to the bonder. The reference values can beobtained using a trial bonding process and can be stored in the EEPROM13.

In response to an error output signal from the apparatus 1, the bondercan download all the individual integrated values from the apparatus 1for further processing and analysis and/or can perform a dummy bond on adummy die pad if necessary.

As an alternative to the apparatus shown in FIG. 1, if a fast ADC 10 isused and a processor 3 which can sample at small time intervals, such asa digital signal processor, it may not be necessary to use theintegrators 8, 9 and the output 5 from the voltage detector may be feddirectly to the ADC 10 and then to the digital signal processor.

An advantage of the invention is that by monitoring the glow dischargevoltage, it is possible to obtain an indication of whether the ballformed is of the correct size. Therefore, it is possible to control thebonder to prevent the bonder attempting to bond a ball to a die pad ifthe ball may be of the incorrect size.

What is claimed is:
 1. A method of monitoring a ball forming process ina wire bonder, the method comprising the steps of: monitoring the glowdischarge between an electrode on the wire bonder and an end of a wiremounted on the wire bonder on which the ball is being formed, comparinga parameter of the glow discharge with a reference value; anddetermining from such comparison whether the ball formed is of a correctsize.
 2. A method according to claim 1, wherein the parameter is theglow discharge voltage.
 3. A method according to claim 1, wherein theparameter is the glow discharge current.
 4. A method according to claim1, further comprising interrupting the automatic bonding process orperforming a dummy bond if the glow discharge parameter differssignificantly from the reference value.
 5. A method according to any ofthe preceding claims, wherein the ball formation process is divided intoa number of time intervals and the glow discharge parameter during eachtime interval is integrated over each time interval, and for each timeinterval each integrated value is compared with a first reference value.6. A method according to claim 1, wherein the mean value of the glowdischarge parameter during the ball formation process is determined andthe mean value is compared with a second reference value.
 7. Apparatusfor monitoring the formation of a bail on an end of a wire in a wirebonder comprising: a detector which is operative to detect a parameterof a glow discharge between an electrode on the wire bonder and an endof a wire during the bail formation of a ball; and a signal processorcoupled to the detectors, the signal processor being operative tocompare an output signal from the detector with a reference value, andto determine from such comparison whether the ball formed is of acorrect size.
 8. Apparatus according to claim 7, further comprising anintegrator to integrate the parameter over a time interval.
 9. Apparatusaccording to claim 8, wherein the glow discharge is divided into anumber of time intervals and the integrator integrates the parameterover each time interval, and the signal processor compares eachintegrated value with the reference value.
 10. Apparatus according toclaim 8, wherein the apparatus comprises two integrators, eachintegrator integrating the parameter over alternate time intervals.